This invention relates to a polarizable electrode for use in an electric double-layer capacitor and, in particular, to a polarizable electrode for use in an electric double-layer capacitor suitable as a capacitor element of an open-air solar generator system. This invention also relates to a method of producing the polarizable electrode mentioned above.
An electric double-layer capacitor has a low resistance and a large capacitance. Because no chemical reaction is involved unlike a battery, deterioration due to repetition of charging and discharging operations is extremely small. Therefore, the electric double-layer capacitor can be used as a maintenance-free capacitor element. In addition, the electric double-layer capacitor is harmless to the environment since any harmful substance such as a heavy metal is not contained in its materials. In view of the above-mentioned advantages, the electric double-layer capacitor is widely used as a memory backup component. Specifically, the electric double-layer capacitor is connected in parallel to a power supply which comprises a battery or a DC power supply implemented by a commercial AC power supply and an AC/DC converter.
Upon occurrence of instantaneous interruption of the power supply, electric charges stored in the electric double-layer capacitor are used to back up various components. Typically, the electric double-layer capacitor used as the memory backup component has a capacitance of 2 to 3 F at most. In recent years, however, development is made of an improved electric double-layer capacitor having a remarkably large capacitance.
Japanese Patent No. 2054380 (corresponding to Japanese Unexamined Patent Publication (JP-A) No. 04-288361 (288361/1992)) discloses a polarizable electrode comprising an activated carbon/carbon composite. The activated carbon/carbon composite is formed by preparing a mixture of activated carbon powder and phenolic resin powder, molding the mixture into a molded product, and subjecting the molded product to heat treatment in an inactive gas atmosphere at a temperature on the order of 900xc2x0 C. to carbonize the phenolic resin. It is reported in the above-mentioned patent that an electric double-layer capacitor having a capacitance of 470 F can be obtained by the use of the polarizable electrode.
Japanese Patent No. 2052267 (corresponding to Japanese Unexamined Patent Publication (JP-A) No. 63-226019 (226019/1988)) discloses a similar polarizable electrode comprising an activated carbon/carbon composite obtained by heating a mixture of activated carbon powder or fiber and a phenolic resin in an inactive gas atmosphere.
Each of the above-mentioned electric double-layer capacitors recently developed has an extremely large capacitance and is therefore expected not only as a memory backup component but also as a substitute battery or an auxiliary battery to back up an overall apparatus or to drive a motor. In particular, in response to a recent demand as a power supply of an electric automobile or a hybrid automobile, development in progress is intended for power applications which require excellent high-current characteristics. For the power applications, it is necessary to reduce a resistance of an electrolytic solution in the activated carbon/carbon composite electrode because such resistance prevents a discharging operation at a high electric current.
Japanese Unexamined Patent Publications (JP-A) Nos. 07-99141 (99141/1995) and 07-201677 (201677/1995) disclose activated carbon/carbon composite electrodes with small pores formed inside so as to improve high-current characteristics. Each of the activated carbon/carbon composite electrodes adapted for the power applications is excellent in high-current characteristics but has a capacitance per unit volume as small as 30-40 F/cm3.
In the meanwhile, fossil resource energy supporting the current human life is not only limited in quantity but also has an adverse influence upon the environment which becomes more and more serious year after year. Under the circumstances, attention is directed to solar energy which is clean and infinite. A solar generator system utilizing the solar energy typically comprises a solar cell for converting the solar energy into electric energy and a capacitor element such as a lead-acid battery for storing the electric energy.
Such a solar generator system is advantageously used in an illumination light or a sign light which is lightened at night. Specifically, the electric energy generated from the solar energy during the daytime is stored in the capacitor element and spent at night to lighten the illumination light or the sign light.
In case where the capacitor element is used in combination with the solar cell to lighten the illumination light or the sign light at night, the capacitor element must perform each of charging and discharging operations once a day. If the lead-acid battery is used as the capacitor element, the lead-acid battery must be exchanged in one or two years because of heavy deterioration resulting from repetition of the charging and the discharging operations, In addition, the lead-acid battery itself is one of the factors: which cause environmental pollution. Taking the above into consideration, it is proposed to use the electric double-layer capacitor in place of the lead-acid battery. The electric double-layer capacitor can store smaller energy as compared with the lead-acid battery but does not require maintenance because of little deterioration resulting from the repetition of the charging and the discharging operations.
However, several problems arise if the electric double-layer capacitor is used in combination with the solar cell to lighten the illumination light or the sign light at night. At first, it is required to store in a single charging operation the electric energy sufficient to lighten the illumination light or the sign light all night. Specifically, an energy density of at least {fraction (1/10)} of that of the lead-acid battery is generally required although it depends upon an overall scale of an illumination facility and an installation space. On the other hand, the above-mentioned electric double-layer capacitor developed for the power applications achieves an energy density on the order of {fraction (1/20)} of that of the lead-acid battery. Thus, in order to use the electric double-layer capacitor in combination with the solar cell in the above-mentioned manner, the energy density must be increased to a level twice that for the power applications.
Second, the illumination light or the sign light are installed in the open air and is therefore exposed to a severe temperature environment. For example, the temperature falls down to xe2x88x9220xc2x0 C. or less in a cold region. In order to allow the use in such an environment, the electric double-layer capacitor is required to be excellent in low-temperature characteristic.
In addition, there is a demand for an improved method of producing a polarizable electrode, which is simple in process and excellent in productivity.
It is an object of this invention to provide a polarizable electrode adapted for use in an electric double-layer capacitor large in capacitance and excellent in low-temperature characteristic.
It is another object of this invention to provide a method of producing a polarizable electrode of the type described, which is simple in process and excellent in productivity.
Other objects of this invention will become clear as the description proceed.
According to a first aspect of this invention, there is provided a polarizable electrode comprising an activated carbon/carbon composite consisting essentially of an activated carbon component and a carbon component, the activated carbon component comprising activated carbon powder, the carbon component being produced by carbonizing a thermosetting resin, the composite having a density within a range between 0.70 and 0.85 g/cm3, both inclusive, and a thickness within another range between 0.7 and 3.0 cm, both inclusive.
According to a second aspect of this invention, the above-mentioned thermosetting resin is at least one selected from a group consisting of a phenolic resin, a furan resin, a urea resin, a melamine resin, a guanamine resin, an unsaturated polyester resin, a diallyl phthalate resin, an allyl diglycol carbonate resin, an epoxy resin, a vinyl ester resin, a phenoxy resin, and a polyurethane resin.
According to a third aspect of this invention, a weight ratio R of the activated carbon powder to the total weight of the activated carbon powder and the thermosetting resin is represented by:
R=aD+b 
(0.80 cm3/gxe2x89xa6axe2x89xa61.00 cm3/g, 0.24xe2x89xa6bxe2x89xa60.29) 
where Dxe2x80x2 represents a packing density of the activated carbon powder in g/cm3.
According to a fourth aspect of this invention, there is provided a method for producing a polarizable electrode comprising an activated carbon/carbon composite consisting essentially of an activated carbon component and a carbon component, the activated carbon component comprising activated carbon powder, the carbon component being produced by carbonizing a thermosetting resin, the composite having a density within a range between 0.70 and 0.85 g/cm3, both inclusive, and a thickness within another range between 0.7 and 3.0 cm, both inclusive, the method comprising the steps of (a) mixing the activated carbon powder and the thermosetting resin to produce a powdery mixture, (b) molding the powdery mixture into a molded product having a predetermined shape, and (c) subjecting be the molded product to heat treatment in a nonoxidizing atmosphere to carbonize the thermosetting resin so that an activated carbon/carbon composite electrode is obtained as the polarizable electrode, wherein:
the molding step is carried out by press-forming under a pressure between 0.1 and 8.0 tonf/cm2, both inclusive.
According to a fifth aspect of this invention, there is provided a method for producing a polarizable electrode comprising an activated carbon/carbon composite consisting essentially of an activated carbon component and a carbon component, the activated carbon component comprising activated carbon powder, the carbon component being produced by carbonizing a thermosetting resin, the composite having a density within a range between 0.70 and 0.85 g/cm3, both inclusive, and a thickness within another range between 0.7 and 3.0 cm, both inclusive, the method comprising the steps of (a) mixing the activated carbon powder and the thermosetting resin to produce a powdery mixture, (b) molding the powdery mixture into a molded product having a predetermined shape, and (c) subjecting the molded product to heat treatment in a nonoxidizing atmosphere to carbonize the thermosetting resin so that an activated carbon/carbon composite electrode is obtained as the polarizable electrode, wherein:
the mixing step is for mixing the activated carbon powder, the thermosetting resin, and a thermoplastic resin binder in a ratio between 30 and 60 wt %, both inclusive, with respect to the total weight of the activated carbon powder and the thermosetting resin to obtain the powdery mixture;
the molding step being carried out by extrusion-molding.
According to a sixth aspect of this invention, there is provided a polarizable electrode comprising an activated carbon/carbon composite consisting essentially of an activated carbon component and a carbon component, the activated carbon component comprising activated carbon fiber, the carbon component being produced by carbonizing a thermosetting resin, the composite having a density within a range between 0.70 and 0.85 g/cm3, both inclusive, and a thickness within another range between 0.7 and 3.0 cm, both inclusive.
According to a seventh aspect of this invention, the above-mentioned thermosetting resin is at least one selected from a group consisting of a phenolic resin, a furan resin, a urea resin, a melamine resin, a guanamine resin, an unsaturated polyester resin, a diallyl phthalate resin, an allyl diglycol carbonate resin, an epoxy resin, a vinyl ester resin, a phenoxy resin, and a polyurethane resin.
According to an eighth aspect of this invention, a weight ratio R of the activated carbon fiber to the total weight of the activated carbon fiber and the thermosetting resin is represented by:
R=aD+b 
(0.80xe2x89xa6axe2x89xa61.00, 0.24xe2x89xa6bxe2x89xa60.39) 
where D represents a packing density of the activated carbon fiber.
According to a ninth aspect of this invention, there is provided a method for producing a polarizable electrode comprising an activated carbon/carbon composite consisting essentially of an activated carbon component and a carbon component, the activated carbon component comprising activated carbon fiber, the carbon component being produced by carbonizing a thermosetting resin, the composite having a density within a range between 0.70 and 0.85 g/cm3, both inclusive, and a thickness within another range between 0.7 and 3.0 cm, both inclusive, the method comprising the steps of (a) mixing the activated carbon fiber and the thermosetting resin to produce a mixture, (b) molding the mixture into a molded product having a predetermined shape, and (c) subjecting the molded product to heat treatment in a nonoxidizing atmosphere to carbonize the thermosetting resin so that an activated carbon/carbon composite electrode is obtained as the polarizable electrode, wherein:
the molding step is carried out by press-forming under a pressure between 0.1 and 8.0 tonf/cm2, both inclusive.
According to a tenth aspect of this invention, there is provided a method for producing a polarizable electrode comprising an activated carbon/carbon composite consisting essentially of an activated carbon component and a carbon component, the activated carbon component comprising activated carbon fiber, the carbon component being produced by carbonizing a thermosetting resin, the composite having a density within a range between 0.70 and 0.85 g/cm3, both inclusive, and a thickness within another range between 0.7 and 3.0 cm, both inclusive, the method comprising the steps of (a) mixing the activated carbon fiber and the thermosetting resin to produce a mixture, (b) molding the mixture into a molded product having a predetermined shape, and (c) subjecting the molded product to heat treatment in a nonoxidizing atmosphere to carbonize the thermosetting resin so that an activated carbon/carbon composite electrode is obtained as the polarizable electrode, wherein:
the mixing step is for mixing the activated carbon fiber, the thermosetting resin, and a thermoplastic resin binder in a ratio between 30 and 60 wt %, both inclusive, with respect to the total weight of the activated carbon fiber and the thermosetting resin to obtain the mixture; the molding step being carried out by extrusion-molding.